The relationship of cell surface IgD to B cell differentiation and triggering will be studied using 5 approaches: 1. Delta chains will be purified from surface and secreted human IgD in order to define the structural difference necessary for membrane attachment. 2. The possibility that IgD binds to another membrane protein (such as Ia) after interaction with antigen will be studied by cross-linking agents, detergent removal, and co-capping. 3. Spin labeling techniques will be used to determine whether IgM and IgD perturb the membrane differently after interaction with ligand. 4. The role of IgD in the primary response to thymus-dependent and thymus-independent antigens will be studied by antibody-mediated blocking of in vitro responses and adoptive transfer of subpopulations prepared on the FACS. 5. The relationship of IgD to the development of long-term memory to IgG will be determined by assessing the relationship between subpopulations of cells bearing IgD-only and IgG in an adoptive transfer system.